1. Field of the Invention
The present invention relates generally to the field of power tools and, more particularly, to a collet lock arrangement for a power tool of the type receiving interchangeable accessory tools.
2. Description of the Prior Art
Various types of rotating power machinery, particularly with regard to certain types of hand-held power tools, require a user to selectively attach an accessory tool thereto such as a drill-bit or a surface treating disk (for example, a polishing pad, a sanding disk, or a grinder tool). Convenient removal and replacement of interchangeable accessory tools is therefore desirable. A selected accessory tool attaches to an end of a rotatable collet shaft which is mechanically driven by power transmitting components within the power tool. Conventionally, a collet lock is often used to lock the collet shaft against movement during removal and replacement of an accessory tool.
In many portable power tools, including air tools and electric motor-driven tools, a nut is provided to axially secure the accessory tool to the rotatable shaft. When changing accessory tools, the nut must be loosened to allow the shaft to release its grip on the accessory tool. Similarly, when a new accessory tool is added, the nut is tightened (preferably with a wrench or the like) within or about the shaft so as to be rotatably axially driven thereby.
During a removal and/or replacement operation, the collet shaft must be prevented from rotating or else it would be impossible to loosen a tightened nut if the latter is free to rotate freely with the shaft to which it is secured.
During loosening of the nut, an operator connects a first wrench to the secured nut and a second wrench to a wrenching portion along the rotatable shaft. A torque is then applied to the nut by rotating the first wrench in a counter-clockwise direction causing the nut to disengage from the rigidly held rotatable shaft.
In the case of portable power tools of substantial weight, the two handed operation described above is a great inconvenience and dangerous. For example, because the operator's two hands are both being put to use in holding the two wrenches, when changing an accessory tool the operator can easily lose his grip on the tool causing the device to fall on the ground or, alternatively, will be unable to create a strong enough torque about the rotatable shaft since the bulk of the torque is unstably counteracted by the great weight of the housing portion of the power tool to which an end of the collet shaft is mechanically connected.
As a result, the two-handed operation described above becomes a three-handed operation, particularly for large, heavy power tools as the operator inevitably is forced to secure the base or housing portion of the power tool in a vise (the third hand) to facilitate the unscrewing of the nut at an opposite end therefrom.
Alternative constructions for securing the accessory tool to the collet shaft other than with a secured nut are also well known. One such construction is a keyless chuck design. While a three-handed operation is unnecessary, a big disadvantage of the keyless chuck is that its use is limited to relatively light-weight portable tools (such as drills which receive interchangeable driver bits) whose mass and shaft rotational speed is small. Portable tools such as die grinder tools characteristically have a high rotational speed (.apprxeq.20,000 RPM) and are subject to high vibrations. Inevitably therefore, a keyless chuck therewith would become loose over time causing the die grinder bit to fall out or break during use.